1. Field of the Invention
The present invention relates to a communication device and, more particularly, to a communication device having a Network Control Unit (NCU) for forming dial pulses and the DC loop of a communication line (the closure of a direct current).
2. Description of the Related Art
A conventional NCU utilizes relays to form dial pulses and coils for closure of a direct current.
The conventional NCU will be described with reference to FIG. 7. As shown in this drawing, a P relay is turned on and off to generate dial pulses. However, the waveforms of the dial pulses are distorted because of the electromagnetic induction of a coil L for closure of the DC loop. To compensate for such distortion, an S relay is used, and is turned on only when the dial pulses are generated, at which time the direct current will not flow through the coil L.
To closure the direct current, the DC loop between a telephone set and a switching system must be correctly formed when a relay switches the DC loop from the telephone set side to a facsimile device side. The coil L forms the DC loop made up of the telephone set and the switching system.
However, the above conventional art utilizes relays and a coil, each of which is a relatively high-cost component and is larger than a semiconductor component, thus increasing the cost and size of the NCU.
To solve such a problem, it is possible to construct an NCU using semiconductor elements. In such an NCU, however, the waveforms of the DC loop and dial pulses are distorted during an off-hook mode.